Adapt bridge(4) to the new if_input() framework.

Adapt bridge(4) to the new if_input() framework.

Move bridge_input() outside of ether_input() in order to duplicate packets
flowing through a bridge port before applying any transformation on mbufs.

This saves a various m_adj(9)/M_PREPEND(9) dances and remove the bridge(4)
hack from vlan(4).

Tested by mxb <mxb AT alumni DOT chalmers DOT se> and kettenis@

ok bluhm@